When the Earth took a turn for the worse in A.D. 536, what was to blame? Anthropology professor Payson Sheets (Anth’67, MA’69) thinks the answer lies in a plastic bag.

In the long and tumultuous history of planet Earth, A.D. 536 was a particularly rough year.

A veil of dust hung high in the stratosphere, filtering out the sun’s warmth and wreaking havoc around the globe for a decade, with the Northern Hemisphere hit hardest. Oceans cooled, evaporation stalled, rain became scarce and crops withered or froze. In northern China, scribes wrote of famine, mass death and loss of faith in the emperor. In Europe, monks described being able to, for the first time, walk “dry footed” across all the rivers and lakes because the water had turned to ice. In Rome, plague set in. In Mexico, commoners set torches to palaces, looted tombs and butchered elites.

“The world became unlivable,” explains anthropology professor Payson Sheets (Anth’67, MA’69), citing written texts, archaeological evidence and tree-ring data all pointing to what has become known as “the mystery of A.D. 536.” “There is no doubt that it happened. The question is what caused it? Was it a volcano? A comet? Historians have been debating it for decades.”

Now, thanks in part to a plastic bag of volcanic ash Sheets brought home from a graduate school field trip to El Salvador in 1969, he and an interdisciplinary team of researchers from across the country believe they are closing in on an answer. They suspect a massive eruption of the Ilopango volcano in El Salvador is likely to blame for the cataclysmic events of A.D. 536. Now they’re digging deep to find out for sure and see what lessons Ilopango can teach us today.

“A massive eruption happens about every 1,000 or 2,000 years, so another one will likely happen in the next few centuries and it will affect economies and culture and climate,” says paleoecologist Robert Dull, a senior research fellow at the Environmental Science Institute at the University of Texas, who is collaborating with Sheets. “To understand how these other events played out is critical for helping anticipate such events in the future.”

Born in Boulder in 1944, Sheets earned bachelor’s and master’s degrees in anthropology at CU-Boulder before heading to the University of Pennsylvania for a doctorate in the early 1970s.

He was excavating an ancient pyramid at a site called Chalchuapa in far western El Salvador in 1969 when he stumbled upon a fine chalky powder. Coming from Colorado where volcanos are rare, he didn’t immediately recognize it as ash. But once he did, questions flooded his mind: Where did it come from? How old was it? How big was the event that caused it? And what was the human toll?

He scooped several cups of ash into a plastic bag, marked it with the location — Structure C-36 — and tucked it in his backpack for the flight home.
In Miami the customs agent also was intrigued with the white powder.

“He took out a big knife and tasted it,” recalls Sheets, clutching the bag of ash that he still keeps in a locked drawer in his office. “He thought he had a big drug bust on his hands.”

The discovery would ignite a lifelong fascination with volcanoes and their impact on ancient cultures, which has guided him from Panama to Costa Rica to Nicaragua, often during times of political strife that make his work a risky endeavor.

In 1971 Sheets used radiocarbon dating of the ash to publish the first estimated date of the Ilopango eruption to somewhere between A.D. 32 and A.D. 448. In subsequent years, he continued to uncover similar samples across western El Salvador, shrouding a wealth of archaeological treasures that hinted at the scope and impact of the disaster.

“It killed every living person, animal, bird, fish and insect in a huge area overnight,” he says, noting that the eruption killed roughly 100,000 people in El Salvador alone and turned the lush jungle to a vacant desert. “The more I have researched that ash, the more I have learned how huge it was.”
But the link to A.D. 536 wouldn’t come until decades later.

The Ilopango sample migrated to the back burner, replaced by other projects. In the late 1970s Sheets served as lead archaeologist on Ceren, a 1,400-year-old Mayan village, which is now a World Heritage Site known as “the Pompeii of the Americas.”

And for much of the 1980s, a ruthless civil war made travel to El Salvador difficult.

“People were slaughtering each other,” Sheets recalls. “I would still go for short visits, but I couldn’t take students or family there for about eight years.”

Fast forward to the 1990s and Dull — then a graduate student working in El Salvador — stumbled upon some of Sheets’ early writings about Ilopango and became intrigued. He reached out to Sheets with an invitation to collaborate.

“Payson is the grandfather of volcanoes and archaeology in this part of the world,” Dull says. “I learned about the event for the very first time from an article he wrote.”

Using samples from trees buried and preserved by the volcanic ash and state-of-the-art radiocarbon dating by John Southon of the University of California, the team worked together to refine the estimated date of Ilopango’s eruption to somewhere around A.D. 536, give or take a few decades.

Then, in 2008, a team of ice core researchers from Copenhagen published a pivotal study reporting that ice cores taken from Greenland and Antarctica showed a significant spike in sulfur around A.D. 536, a phenomenon that likely resulted from “a large, explosive, near equatorial volcanic eruption.”
Sheets and his colleagues began to connect the dots. Was their volcano the culprit?

“There is plenty of circumstantial evidence but there is no smoking gun yet,” says Sheets, who is still reluctant to say definitively whether Ilopango caused the events of A.D. 536. “But we are going after it.”

As his colleagues in the natural sciences toil to come up with a more precise date of the Ilopango eruption and determine whether it is linked to the cataclysmic events of A.D. 536, Sheets hopes to learn more about the eruption’s cultural impacts.

“What societies were resilient and what societies were not?” he asks.

For now, what they can say for sure is this: Ilopango was likely the biggest volcanic eruption in Central America in the last 84,000 years, and among the most powerful in the history of the planet.

And the events of A.D. 536 — whatever their cause — forever changed life on Earth.

“It is mind-boggling to think about the potential impacts it had on cultures all around the globe,” Dull says. “It was a real pivot point in human history.”